In olefin epoxidation, a feed containing an olefin and an oxygen source is contacted with a catalyst under epoxidation conditions. The olefin is reacted with oxygen to form an olefin oxide. A product mix results that contains olefin oxide and typically unreacted feed and combustion products, such as carbon dioxide.
The catalyst comprises silver, usually with one or more additional elements deposited therewith, on a carrier, typically an alpha-alumina carrier. The olefin oxide may be reacted with water to form a 1,2-diol, with an alcohol to form a 1,2-diol ether, or with an amine to form an alkanolamine. Thus, 1,2-diols, 1,2-diol ethers, and alkanolamines may be produced in a multi-step process initially comprising olefin epoxidation and then the conversion of the formed olefin oxide with water, an alcohol, or an amine.
The performance of the silver containing catalyst may be assessed on the basis of selectivity, activity, and stability of operation in the olefin epoxidation. The selectivity is the molar fraction of the converted olefin yielding the desired olefin oxide. Stability refers to how the selectivity and/or activity of the process changes during the time a charge of catalyst is being used, i.e., as more olefin oxide is produced.
Various approaches to improving the performance of the silver catalysts, including improvements in selectivity, activity, and stability, have been investigated. For example, modern silver-based catalysts may comprise, in addition to silver, one or more high-selectivity dopants, such as components comprising rhenium, tungsten, chromium, or molybdenum. High-selectivity catalysts are disclosed, for example, in U.S. Pat. No. 4,761,394 and U.S. Pat. No. 4,766,105. U.S. Pat. No. 4,766,105 and U.S. Pat. No. 4,761,394 disclose that rhenium may be employed as a further component in the silver containing catalyst with the effect that the initial, peak selectivity of the olefin epoxidation is increased.
Depending upon the catalyst used and the parameters of the olefin epoxidation process, the time required to reach the initial, peak selectivity, that is the highest selectivity reached in the initial stage of the process, may vary. For example, the initial, peak selectivity of a process may be achieved after only 1 or 2 days of operation or may be achieved after as much as, for example, 1 month of operation. EP-A-352850 also teaches that the then newly developed catalysts, comprising silver supported on alumina carrier, promoted with alkali metal and rhenium components have a very high selectivity.
As another example of an approach to improving the performance of the silver catalysts, fluorine has been incorporated into carriers used to prepare epoxidation catalysts, with an intention that the resultant fluoride-mineralized carriers will have morphological properties conducive to improved catalyst performance. The crush strength or attrition resistance of such fluoride-mineralized carriers, however, can often be inherently lower than desirable. While various additives, often referred to as binders, have been used to improve the crush strength or attrition resistance of carriers, traditional binders typically must be subjected to a high temperature treatment to activate their binding properties. Often, the high temperature treatment involves temperatures in excess of 1,200° C., even in excess 1,300° C. The use of such traditional binders with fluoride-mineralized carriers may not be desirable, as the morphology of the fluoride-mineralized carrier may be detrimentally affected if the carrier is exposed to such high temperatures.
Thus, notwithstanding the improvements already achieved, there is a desire to improve the performance of olefin epoxidation catalysts and, in particular, to increase the crush strength or attrition resistance of fluoride-mineralized carriers without detrimentally affecting the morphology of such carriers.